Process for manufacturing high-sensitivity accelerometric and gyroscopic integrated sensors, and sensor thus produced

ABSTRACT

A movable mass forming a seismic mass is formed starting from an epitaxial layer and is covered by a weighting region of tungsten which has high density. To manufacture the mass, buried conductive regions are formed in the substrate. Then, at the same time, a sacrificial region is formed in the zone where the movable mass is to be formed and oxide insulating regions are formed on the buried conductive regions so as to partially cover them. An epitaxial layer is then grown, using a nucleus region. A tungsten layer is deposited and defined and, using a silicon carbide layer as mask, the suspended structure is defined. Finally, the sacrificial region is removed, forming an air gap.

TECHNICAL FIELD

The invention relates to a process for manufacturing high-sensitivityaccelerometric and gyroscopic integrated sensors and a sensor thusproduced.

BACKGROUND OF THE INVENTION

As is known, using electromechanical microstructures of semiconductormaterial, the manufacture of which utilizes microelectronics techniques,has recently been proposed for producing accelerometers and gyroscopes.These silicon micro-machining techniques make it possible to producedifferent types of angular velocity and acceleration sensors. Inparticular, at the present time prototypes operating according to thepiezoelectric, piezoresistive, capacitive, threshold, resonant andtunnel effect principles have been proposed.

Reference will be made below to an accelerometric sensor of differentialcapacitive type, i.e. one in which acceleration induces the movement ofa seismic mass which constitutes the electrode common to two coupledcapacitors by varying the two capacitances in opposite directions. Thiseffect is known as differential variation of capacitance.

Historically, integrated micro-structures have been manufactured bypreferably using the “bulk micro-machining” technique in which a waferof single-crystal silicon is machined on both faces. This technique is,however, incompatible with the process steps for producing components ofa circuit which processes a signal picked up by a sensitive element, asrequired at present.

It has been proposed to use the technique of “surface micro-machining”in which the sensitive element is made of multi-crystal silicon andsuspended structures are formed by depositing and successively removingsacrificial layers. This technique is compatible with the currentintegrated circuit manufacturing processes and is therefore preferred atpresent. The integrated micro-structures produced with this techniqueare, however, relatively insensitive to acceleration and angularvelocity. In fact, having a mass of the order of a few tenths of amicrogram, they suffer the effects of thermodynamic noise caused by theBrownian movement of the particles of the fluid in which they areimmersed (see, for example, the article by T. B. Gabrielson entitled“Mechanical-Thermal Noise in Micromachined Acoustic and VibrationSensors“, IEEE Transactions on Electron Devices, vol. 40, No. 5, May1993). The upper limit to the mass obtainable with these structures isimposed by genuinely technological reasons; the deposition of very thickfilms involves extremely long wafer machining times and renders thesurface of the wafer unsuitable for the successive operations such aslapping the wafers.

A technique for machining the epitaxial layer (epitaxialmicro-machining) is also known, which produces microstructures withinertial masses that are higher and hence more sensitive, but not yet ata sufficient value for practical applications.

SUMMARY OF THE INVENTION

An object of the invention is to improve a process for manufacturing anaccelerometric and gyroscopic sensor according to a technique of“epitaxial micro-machining” so as to increase its sensitivity furtherthan the prior art.

An embodiment of the invention provides a process for manufacturing ahigh-sensitivity accelerometric and gyroscopic integrated sensorincluding forming a sacrificial region on a substrate of semiconductormaterial, growing an epitaxial layer that includes tungsten on thesubstrate and the sacrificial region, and then removing selectiveportions of the epitaxial layer and the sacrificial region to form amovable mass. The moveable mass formed is surrounded at the sides andseparated from fixed regions by trenches, and separated from thesubstrate by an air gap.

It also provides for an accelerometric integrated sensor, having asubstrate and an epitaxial layer of semiconductor material, whereby theepitaxial layer includes tungsten and forms a movable mass which issurrounded at sides by a fixed mass. The movable mass is separated fromthe substrate by a gap from below and from the fixed mass by trenches atthe sides, and is supported by the fixed mass through anchorageportions.

BRIEF DESCRIPTION OF THE DRAWINGS

For an understanding of the invention, a number of preferred embodimentswill now be described, purely by way of non-exhaustive example, withreference to the accompanying drawings.

FIGS. 1-8 show cross sectional views at different points of asemiconductor wafer during successive steps of the manufacturing processaccording to the invention.

FIG. 9 shows a cross sectional view of a plane perpendicular to FIG. 8.

FIG. 10 shows a perspective view of the sensor obtained with the processof FIGS. 1-9.

FIG. 11 shows a top view of the sensor of FIG. 10.

FIGS. 12 and 13 show transverse sections of a portion of a wafer in twosuccessive manufacturing steps according to a different embodiment ofthe process.

DETAILED DESCRIPTION

An embodiment of a capacitive-type accelerometric or gyroscopic sensoraccording to a first embodiment of the process will now be describedwith reference to FIGS. 1-10, in which the thicknesses of the variouslayers of material are not to scale and some layers are not shown in allthe illustrations for reasons of representation.

Shown in FIG. 1, buried N⁺-type conductive regions 2, 3 to form buriedinterconnections are formed in a substrate 1 of single-crystal siliconof P-type conductivity, using conventional masking and implantationtechniques. A pad oxide layer 5 is formed, e.g., grown thermally, on asurface 4 of the substrate 1, and a silicon nitride layer 6 is depositedon it. The silicon nitride layer 6 is then defined and removedselectively in a sensor zone 7. Then the portions of the surface of thesubstrate 1 not covered by the layer 6 are locally oxidated, formingoxide regions comprising a sacrificial region 8 (surrounded at the sidesand underneath by the buried conductive region 3) and buried oxideregions 9a, 9b, 9c and 9d at the buried conductive region 2, obtainingthe structure of FIG. 2.

Through suitable masking steps, portions of the layers 5, 6 are thenremoved in the sensor zone 7 where the buried contacts of the sensor andof the silicon nitride layer 6 are to be formed in the circuitry andfrom an interconnection area 10, obtaining the structure of FIG. 3. InFIG. 3, the pad oxide layer 5 underneath the silicon nitride layer 6 isnot shown and 6a, 6b and 6c denote the portions of nitride included,respectively, between the buried oxide regions 9a and 9b; 9b and 9c andthe regions 9d and 8.

An amorphous or multi-crystal silicon layer 12 is then deposited, asshown in FIG. 4. By means of a phototechnique and plasma etching step,the amorphous or multi-crystal silicon layer 12 is removed, except inthe sensor zone 7, forming a silicon region 12′ representing the nucleusfor a successive epitaxial growth step. By means of chemical etching,the pad oxide layer 5 is then removed where exposed and epitaxial growthtakes place with formation of a “pseudo-epitaxial”, P-type layer 13. Inthe sensor zone 7, the layer 13 has a multi-crystal structure(multi-crystal region 13′) and a single-crystal structure elsewhere(single-crystal region 13″). A wafer 14 as shown in FIG. 5 is thusobtained.

The pseudo-epitaxial layer 13 is then doped with doping ions suitablefor determining an N-type conductivity to form deep regions. Inparticular, as shown in FIG. 6, in which a portion of the wafer 14 isshown slightly displaced to the left with respect to FIGS. 1-5, a deepN⁺-type region 18 is formed in the single-crystal region 13″ and extendsfrom a surface 16 as far as the buried conductive region 2. This deepregion 18 electrically connects the buried conductive region 2 to thesurface 16. Also formed in the multi-crystal region 13′ is an N⁺-typewell 19 that extends from the surface 16 as far as the buried conductiveregion 3 (see FIG. 7) and, partially, the buried conductive region 2. Inparticular, the well 19 extends above the buried oxide regions 9c, 9dand half of the buried oxide region 9b, electrically contacting theburied conductive region 2 in the area included between the buried oxideregions 9c and 9d that are not covered by the portions of nitride 6a-6c.

The electronic components of the circuitry are then formed by means ofstandard steps. In the example shown, an N-type collector well 15 isformed, extending from the surface 16 of the pseudo-epitaxial layer 13as far as the substrate 1. An NPN transistor 23, having an N⁺-typecollector contact region 20, a P-type base region 21 and an N⁺-typeemitter region 22 is formed in the collector well 15.

A dielectric layer 24 for opening the contacts, e.g., BPSG (boronphosphorus silicon glass) is then deposited on the surface 16 of thewafer 14. Then, by a suitable masking and selective removal step, thecontacts are opened in the circuitry area and on the deep region 18, anda part of the dielectric layer 24 is removed from the sensor zone 7. Anadhesive layer 25 (of titanium nitride for example) is then deposited,to facilitate the adhesion of the next layer to the silicon of the wafer14. A tungsten layer 26 is deposited by CVD (Chemical Vapor Deposition)at a thickness of 1 μm thick, for example, obtaining the intermediatestructure of FIG. 6. The nucleus silicon region 12′ has been omitted inFIG. 6.

The tungsten layer 26 is then shaped, by means of knownphotolithographic steps, so as to form contacts 26a of the circuitry and26b of the sensor and a weighting region 26c over the well 19, as shownin FIG. 7 in which the adhesive layer 25 is not shown. In particular,the weighting region 26c is shaped as partially shown in FIG. 10, i.e.,corresponding to the shape of the movable electrode of the sensor, asexplained in greater detail below. A dielectric passivation layer 30 isthen deposited and this is removed in the zone of the contact pads (topermit the electrical contacting of the device, in a manner not shown),and in the sensor zone 7, thus obtaining the structure of FIG. 7.

A silicon carbide layer 31, intended to form a mask for the subsequentstep of excavation of the pseudo-epitaxial layer 13 and precisely of themulti-crystal region 13′, is then deposited and defined. Excavations arecarried out to release the movable mass of the accelerometer, toseparate the fixed and movable electrodes and to insulate the regions atdifferent potential. Thus a trench 33a which separates the fixed partfrom the movable part and the fixed mass from the surrounding portion ofthe well 19 is formed. A trench 33b (see FIGS. 10 and 11) separating theanchorage regions from the surrounding portion of the well 19 and atrench 33c separating the sensor from the rest of the chip are alsoformed. The structure is thus obtained which is shown in FIG. 8, takenon the same section as FIGS. 1-7 but centred on the sensor zone 7, andin FIG. 9, taken perpendicular to that of FIG. 8. FIG. 9 showstransverse walls 34 and 35 defining the movable electrodes and the fixedelectrodes of the sensor, as explained in greater detail below withreference to FIGS. 10 and 11.

Finally, the sacrificial region 8 is removed by etching in, e.g.,hydrofluoric acid, and the zone previously occupied by this region 8forms an air gap 38 which at the bottom separates the movable mass fromthe rest of the wafer. The movable mass is then etched and supported bythe chip only at the anchorage zones. With a subsequent etching inplasma, the silicon carbide layer 31 is removed from all areas of thewafer. The final structure is thus obtained which is shown in FIGS. 10and 11 in which the movable mass is denoted by 40, the fixed mass by 41,and the anchorage zones of the movable mass by 42. In particular, FIG.11 shows the outer edge of the buried conductive region 3 in brokenlines and the outer edge of the well 19 in dot-and-dash lines. Brokenlines also denote the buried conductive regions 2 for forming the buriedconnections of the fixed mass, and 2′, 2″ those of the movable mass,formed at the same time and in the same way as the buried conductiveregion 2. FIG. 10 also shows the profile of the weighting region 26c.

As will be noted, the movable mass 40 is H-shaped and the transversewalls 34 define the movable electrodes of the capacitive sensor. Themoveable electrodes are interleaved in a comb-like manner with thetransverse walls 35 defining the fixed electrodes and are separated fromits central element. The structure is therefore equivalent to acapacitor formed by two capacitors in series, each formed by a pluralityof elementary capacitors connected in parallel.

In per se known manner, through the deep regions 18 and the buriedconductive regions 2, 2′, 2″, and 3, the movable electrodes 34 and thefixed electrodes 35 are biased at different voltages so that when themovable mass 40 is subjected to acceleration, the consequent change ofdistance between the movable electrodes and the fixed ones may bedetected as a variation of capacitance.

Manufacturing the movable mass 40 in a semiconductor material having atungsten weighting region 26c, as described, gives the sensor highsensitivity. In fact, tungsten has high density (19.3 g/cm3) withrespect to multi-crystal or amorphous silicon (2.33 g/cm3).Consequently, a tungsten layer 1 μm thick is virtually equivalent, fromthe point of view of the mechanical properties, to a 10 μm polysiliconlayer. On the other hand, the deposition by CVD of a tungsten layer ofthe indicated thickness can easily be achieved with the conventionalintegrated microelectronics machining techniques.

The sensor obtained in this way thus has high sensitivity, yet benefitsfrom the advantages typical of epitaxial machining technology andpermits the integration of the sensor together with the integratedsignal processing circuit.

The manufacturing process is simple to implement, using steps typical ofmicroelectronics and forms the metallic circuit interconnection regionsand the weighting regions of the movable structure at the same time. Theprocess is also readily controllable and repeatable.

According to a different embodiment of the invention, the buried oxideregions 8 and 9 are grown in recesses previously formed in the substrate1, after the buried conductive regions 2, 3 have been formed. In detail,shown in FIG. 12, starting from the structure of FIG. 1, the oxide 5 andnitride 6 layers are formed and defined in a similar manner to thatdescribed with reference to FIG. 2. The portions of substrate 1 notcovered by the layers 5, 6 are then etched, forming recesses 50 (FIG.12). The recesses 50 are then filled with thermally grown oxide regions,only the sacrificial region 8′ and the buried oxide region 9d′, shown inFIG. 13. The further steps described above then follow, starting fromthe removal of portions of nitride 6 and of oxide 5 where the contactsare to be formed and in the zone of the circuitry, as described fromFIG. 3 onwards.

According to a further embodiment which is not shown, the sacrificialand buried oxide regions may be obtained by depositing and shaping anoxide layer.

Finally it will be clear that numerous modifications and variations maybe introduced to the process and sensor described and illustratedherein, all coming within the scope of the inventive concept as definedin the accompanying claims. In particular, the components of thecircuitry integrated with the sensor may be either bipolar or MOS; theconductivity of the conductive regions may be the opposite of that shownand the protective and/or adhesive materials may be replaced by otherswhich are equivalent as regards the functions desired, as well as otherchanges and variations.

1. An integrated sensor, comprising: a substrate of a first conductivitytype and an epitaxial layer of semiconductor material formed on saidsubstrate, said epitaxial layer forming a movable mass which that issurrounded at its sides by a fixed mass; said movable mass beingseparated from said substrate by a gap and at the sides from said fixedmass through trenches formed in said epitaxial layer; said movable massbeing supported by said fixed mass through anchorage portions in saidepitaxial layer; and buried conductive regions of second conductivitytype formed in the substrate and providing electrical contact betweenthe movable mass and the fixed mass, the buried conductive regionsselectively facing the epitaxial layer; and a weighting regioncomprising tungsten at said movable mass.
 2. The sensor according toclaim 1 wherein said weighting region extends above said movable mass.3. The sensor according to claim 2 wherein said weighting region issurrounded by a protective layer of silicon carbide.
 4. The sensoraccording to claim 2, further comprising electronic components formed ina single-crystal epitaxial region in said epitaxial layer wherein saidelectronic components comprise tungsten contact electrodes.
 5. Thesensor according to claim 4 wherein adhesive titanium nitride regionsextend underneath said weighting region and said contact electrodes. 6.The sensor of claim 1 wherein said substrate has a first conductivitytype, the sensor , further comprising: buried conductive regions of asecond conductivity type extending in said substrate and selectivelyfacing said epitaxial layer; electrically insulating material regionsextending on said buried conductive regions and delimiting therebetweenportions of selective contact between said buried conductive regions andsaid movable mass and said fixed mass; and deep contact regionsextending from a surface of said epitaxial layer as far as said buriedconductive regions to form deep contacts.
 7. The sensor of claim 1wherein said movable mass has movable electrodes facing and interleavedwith fixed electrodes extending from said fixed mass to form a sensor ofcapacitive type, said movable electrodes comprising respective tungstenweighting regions.
 8. An integrated sensor comprising: a semiconductorsubstrate having an epitaxial layer formed thereon; a fixed mass formedin the epitaxial layer on the substrate; and a movable mass formed inthe epitaxial layer and suspended over the substrate to form a gapbetween the movable mass and the semiconductor substrate, the movablemass including a tungsten layer and being supported by the fixed massthrough anchorage portions and separated from the fixed mass bytrenches; buried conductive regions of a second conductivity typeextending in said substrate and selectively facing said epitaxial layer;electrically insulating material regions extending on said buriedconductive regions and delimiting therebetween portions of selectivecontact between said buried conductive regions and said movable mass andsaid fixed mass; and a weighting region.
 9. The sensor of claim 8wherein the movable mass includes a tungsten layer is surrounded by aprotective layer.
 10. The sensor of claim 8, further includingelectronic components formed on the fixed mass.
 11. The sensor of claim8 wherein the movable mass has moveable electrodes interleaved withfixed electrodes that extend from the fixed mass thus forming acapacitive sensor.
 12. An integrated sensor, comprising: a substratehaving a first conductivity type and an epitaxial layer of semiconductormaterial, the epitaxial layer configured to have a movable masssurrounded at its sides by a fixed mass, the movable mass beingseparated from the substrate by a gap and separated at the sides fromthe fixed mass through trenches, the movable mass being supported by thefixed mass through anchorage portions in the epitaxial layer; aweighting region comprising tungsten at the movable mass; buriedconductive regions of a second conductivity type extending in thesubstrate and selectively facing the epitaxial layer; electricallyinsulating material regions extending on the buried conductive regionsand delimiting therebetween portions of selective contact between theburied conductive regions and the movable mass and the fixed mass; anddeep contact regions extending from a surface of the epitaxial layer asfar as the buried conductive regions to form deep contacts.
 13. Thesensor of claim 12 wherein the weighting region comprises tungsten thatextends above the movable mass.
 14. The sensor of claim 13 wherein theweighting region is surrounded by a protective layer of silicon carbide.15. The sensor of claim 13, further comprising electronic componentsformed in a single-crystal epitaxial region in the epitaxial layer, andthe electronic components comprising tungsten contact electrodes. 16.The sensor of claim 15 wherein adhesive titanium nitride regions extendunderneath the weighting region and the contact electrodes.
 17. Anintegrated sensor, comprising: a substrate of a first conductivity typeand an epitaxial layer of semiconductor material formed on saidsubstrate, said epitaxial layer forming a movable mass that issurrounded at its sides by a fixed mass; said movable mass separatedfrom said substrate by a gap and at the sides from said fixed massthrough trenches formed in said epitaxial layer; said movable masssupported by said fixed mass through anchorage portions in saidepitaxial layer; buried conductive regions of a second conductivity typeextending in the substrate and selectively facing the epitaxial layer;electrically insulating material regions extending on the buriedconductive regions and delimiting therebetween portions of selectivecontact between the buried conductive regions and the movable mass andthe fixed mass; and a weighting region comprising a metal at saidmovable mass.
 18. The integrated sensor of claim 17 wherein saidweighting region comprises the metal layer formed on top of the fixedmass.
 19. The integrated sensor of claim 17, the sensor furthercomprising: deep contact regions extending from a surface of theepitaxial layer as far as the buried conductive regions to form deepcontacts.
 20. An integrated sensor, comprising: a substrate having afirst conductivity type and an epitaxial layer of semiconductor materialformed on said substrate, said epitaxial layer forming a movable massthat is surrounded at its sides by a fixed mass; said movable mass beingseparated from said substrate by a gap and at the sides from said fixedmass through trenches formed in said epitaxial layer; said movable massbeing supported by said fixed mass through anchorage portions in saidepitaxial layer; a weighting region at said movable mass; buriedconductive regions of a second conductivity type extending in saidsubstrate and selectively facing said epitaxial layer; electricallyinsulating material regions extending on said buried conductive regionsand delimiting therebetween portions of selective contact between saidburied conductive regions and said movable mass and said fixed mass; anddeep contact regions extending from a surface of said epitaxial layer asfar as said buried conductive regions to form deep contacts.
 21. Thesensor according to claim 20 wherein said weighting region extends abovesaid movable mass.
 22. The sensor according to claim 21 wherein saidweighting region is surrounded by a protective layer of silicon carbide.23. The sensor according to claim 21, further comprising electroniccomponents formed in a single-crystal epitaxial region in said epitaxiallayer wherein said electronic components comprise tungsten contactelectrodes.
 24. The sensor according to claim 23 wherein adhesivetitanium nitride regions extend underneath said weighting region andsaid contact electrodes.
 25. The sensor of claim 20 wherein said movablemass has movable electrodes facing and interleaved with fixed electrodesextending from said fixed mass to form a sensor of capacitive type, saidmovable electrodes comprising respective tungsten weighting regions.